When plant and animal species wind up where they don't belong, they can attack ecosystems and economies with terrible consequences.

Get a taste of what awaits you in print from this compelling excerpt.

In the hour of long shadows and cooling tarmac, the snakes of the Everglades slip out in search of supper. Cruise control set at 25, my cherry red sports car slips out after them, prowling ceaselessly back and forth, back and forth, on the two-lane river of tar that runs through the park.

At 8:23 a short, fat snake appears, immobile, in my lane. I leap out to examine the stubby little creature andTssssss!it flicks its head straight back and snaps its jaws open, presenting a sinister flower of petal pink flesh. Yikes. A cottonmouth. I hop back in the car. At 8:28 a dark, sinuous snake, slim as a bootlacetoo slim to be what I'm looking for. At 9:03 another little fatty, worth a closer look. Nope. A pygmy rattlesnake, maybe.

Then a dry spell. At 10:00, headlights appear behind me. I watch them coming on fast in the rearview mirror and when my gaze returns to the road ahead, there's my snakea roadblock of a snake, as thick as a thigh and as long as the lane is wideand I'm almost on top of it. I mash the brake pedal and fling out an arm to warn the driver behind me. The vehicle swerves around me. An instant before contact, the driver sees the snake, lurches onto the shoulder, lurches back onto the road, and speeds away. The glossy argyle of bronze and charcoal lies unscathed.

Python molurus bivittatus, the Burmese python, is a species you won't find in any field guide of the endemic reptiles of North America; it's a native of Southeast Asia. But anyone in Florida who wants to see one in the wild can try the Everglades National Park road on a summer's eve. It's a bizarre sight: The guy in my headlights is already bigger than any other snake in North America, yet it's a mere pipsqueak by its own standards. It may live 25 years and reach 20 feet (6 meters) in length. It can achieve the girth of a telephone pole; it can dine on full-grown deer.

Skip Snow, a park biologist, has examined scores of Burmese pythons found in the Everglades in the past few years, hatchling, juvenile, and adult. "There's little doubt they've become established and are breeding here," he told me, though you could see he still has trouble believing it himself.

If Snow were here tonight, he'd pin this ten-footer (three-meter) with a snake stick, wrestle it into a rubber tub, and run it back to the lab where he'd euthanize and autopsy it. But Snow is busy elsewhere, and yours truly is cowering in the sports car, fighting an absurd urge to lock the doors. The animal lies still just long enough for me to get a good look, then glides heavily into the scrub.

In More to Explore the National Geographic magazine team shares some of its best sources and other information. Special thanks to the Research Division.

"Experts fear 'Frankenfish' has terrorized harbor," a Chicago headline blared last October 15. Poster child of invasive species, a northern snakehead fish had been netted in Lake Michigan on Chicago's shoreline. The Asian alien surfaced in the heart of the city's scientific community near the Adler Planetarium, the Field Museum, and the Shedd Aquarium.

"I think it was almost certainly released from a home aquarium," says Field Museum biologist Philip Willink, who first positively identified the snakehead. Willink preserved the 18-inch (46-centimeter) specimenone newspaper fish story said it was a 40-incher (100-centimeter)for the museum's collection.

Northern snakeheads can grow to about three feet (one meter) in their native waters in China, and perhaps Korea and Russia. Voracious predators with a business end full of sharp teeth, the fish can also survive out of water for hours and can wriggle on land for short distances. The fish first made nationwide news in 2002 when a pair and hundreds of their young were discovered in a Maryland pond. All were poisoned, but months later more snakeheadsunrelated to those in the pondmysteriously began turning up in the Potomac River near Washington, D.C. At least 19 have now been caught, including a baby snakehead that confirms a breeding population in the Potomac.

"There is no clue about where the Potomac fish came from. The restaurant industry is the best guess," says John Odenkirk, a biologist for the Virginia Department of Game and Inland Fisheries. Despite speculation that snakeheads might destroy an ecosystem by devouring all other species, such as largemouth bass, Odenkirk cautions that "all we have is the big unknown. It remains to be seen how the newcomer will fit into the Potomac's ecology."

The ecological experiment has now widened. Northern snakeheads have also reared their heads in California, Florida, and Massachusetts. Several were found last July in Philadelphia waterways. In Chicago, Willink of the Field Museum devoutly hopes that Illinois doesn't join the list. If the October snakehead wasn't dumped into the harbor, there is a more ominous possibilitythe Chicago River, linked to both Lake Michigan and the Illinois River.

"If snakeheads come up the Illinois River watershed and invade Lake Michigan, they could impede some fisheries like bass, perch, and sunfish," says Willink. Authorities are already worried about another invasive species on their doorstepthe Asian, or bighead, carp. Weighing up to 100 pounds (50 kilograms) and a potential menace to the Great Lakes' native fish, the carp have swum north through the Illinois River to within 50 miles (80 kilometers) of Chicago. Construction has just begun on a nine million dollar underwater electric fence in a canal feeding the Chicago River in an attempt to keep the carp from reaching Lake Michigan. The fish can also leap more than eight feet (two meters) from the water when startled and have struck several boaters.

John L. Eliot

Global Invasive Species Databasewww.issg.org/database/welcomeSearch this database for invasive plants, animals, and microorganisms by country, habitat, or taxonomic group. Expert contributors from around the world provide information on species biology, ecology, native and alien ranges, links, and images in an easy-to-navigate website. The Global Invasive Species Database was developed by the Invasive Species Specialist Group, part of the World Conservation Union (IUCN).

National Invasive Species Councilwww.invasivespecies.gov/geog/state/statemain.shtmlThe National Invasive Species Council coordinates the federal government's responses to invasive species in the United States. Search this extensive website by state, species, and type and find links to government agencies and nonprofit organizations that lead to even more information.